Electronic timing circuit



H. MERRILL.l

Filed Aug. 17, 1945 ELECTRONIC TIMING CIRCUIT Feb. 2l, 1950 Patented Feb. 21, 1950 UNITED STATES PATENT OFFICE Seeburg Corporation tion of Illinois Chicago, Ill., a corpora- Application August 17, 1945, Serial No. .611,187

(Cl. Z50-27 7 Claims.

This invention relates to timing circuits and has for its principal object to provide a timing circuit which will give uniform results with a high degree of accuracy notwithstanding wide variations of voltage in the power supply.

The present invention is particularly adapted for use with bombing controls. A manual actuation initiates the opening of the bomb bay doors and the operation of the timing circuit. At the end of its time period the circuit initiates the operation of the intervalometer or other bomb dropping mechanism. The accuracy of bomb placement depends upon the accuracy of the timing circuit. Error ought to be less than .05 second in a 4 second interval, and this accuracy must be maintained although the voltage, normally provided by storage batteries, may vary between 25 and 30 volts.

The timing circuit hereinafter described and claimed exceeds these requirements. While it is particularly well adapted for the specic purpose referred to, it is, of course, capable of use for timing purposes generally.

The invention will readily be understood from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawing which is a wiring diagram thereof.

Referring to the drawing, the reference numeral I designates the positive line and the reference numeral 2, the negative line. The start button 3, when actuated manually, closes the switch 4, of the first control circuit, which may for example initiate the opening of the bomb bay doors. The button 3 also closes the switch 5 energizing the relay 6 which closes the contacts 1. The relay 8 is closed and is locked up by the normally open contacts 9 in series with the normally closed contacts IIJ.

A contact II connected to the negative line 2 normally engages a contact I 2 which is connected to the line I3. When the relay 8 is energized this engagement is broken and the contact II engages the contact I4 which is connected to the line I5. The line I is connected to the lefthand side of the condenser I6, through the re-` sistor II to the line I, and to the cathodes I8 of the double tetrode I9. The line I3 is connected to the right-hand side of the condenser I6, through the resistor 20 to the line I, and to the control grid of the upper tetrode. The control grid of the lower tetrode is connected through a C battery 2l to the movable arm of a voltage divider 22 whose resistor is connected between the lines I and 2. The screen grids of the two tetrodes vthan the line I.

and the plate of the lower tetrode are connected through a resistor 23 to the line I. The

Aplate of the upper tetrode is connected through the winding of the relay 24 to the line I. The relay 24 controls the normally closed contacts I0 and the normally open contacts 25 which are in the second control circuit which controls the operation which is to be performed at a certain time after the actuation of the button 3.

The tube employed may suitably be a 28D'7 tube and the condenser I6 may suitably be a 10 microfarad condenser. For a four second period the resistor 20 may suitably have a value of 740,000 ohms. Suitable Values for the resistors Il, 23 and 22, are 10,000, 2,400, and 1,000 respectively. The battery 2| is suitably a ve cell battery applying approximately 71A; volts negative bias to the lower control grid.

Normally the left-hand side of the condenser I6 is positive and the right-hand side is negative. When the start button 3 is actuated, the relays 6 and 8 are energized and the latter is locked up by the contacts 9 and I0. The closing of the switch 4 starts the first operation. When the button 3 is actuated the left-hand side of the condenser I6 is connected to the negative line 2 by the contacts II and I4 and the right-hand side of the condenser I6 is connected to the positive line I through the high resistance 20. The charge on the condenser is thus reversed gradually and the voltage on the line I3 builds up in the positive direction and the plate current through the relay 24 builds up until it is strong enough to actuate the relay. If the voltage across lines I and 2 is low it will take a longer time for the plate current to build up to relay actuating strength, and vice versa. The upper tetrode is therefore compensated by a voltage derived from the lower tetrode. This voltage is derived from the output circuit of the lower tetrode and is applied to the upper screen grid. The lower tetrode provides a voltage which increases the plate current of the upper tetrode when the voltage across lines I and 2 is low and vice versa, as will now be described.

The voltage on the lower control grid is the algebraic sum of the negative voltage provided by the battery 2I and the positive voltage provided by the voltage divider 22. When the relay 8 is energized the cathodes are connected to line 2. Consequently the lower tetrode provides plate current and there is a certain voltage drop across the resistor 23. Thus the screen grid of the upper tetrode has a somewhat lower voltage When the voltage across lines l and 2 is low the voltage of the control grid of the lower tetrode is more negative. There is a smaller plate current and the voltage of the upper screen grid is higher. As has been pointed out above the lower voltage across lines l and 2 tends to extend the period before the energization of the relay 24 and the higher voltage applied to the upper vscreen grid tends to compensate by increasing plate current of the Yupper tetrode. The degree of compensation can be adjusted to the optimum by means of the voltage divider 22. The Value of the resistor y20 controls the period for which the timing circuit is set and this period may be adjusted by adjustment of the resistor.

Thus notwithstanding wide variations in voltage between the lines I and 2 I can attain the desired timing period, for example 4 seconds consistantly with an error of considerably less than .05 second. y

The employment of a single envelopecontaining two tetrodes is merely incidental. Any equivalent thermionic tube 'or tubes may be employed. The fundamental feature of the invention is that I have one thermionic means associated with the timing circuit vand subject to error resulting from variation of Vthe vsupply voltage. To correct this error I derive a corrective voltage from another thermionic means which voltage also varies with the supply voltage and I supply this corrective voltage tothe first thermionic means so as to get a standard timing period.

Although the invention has been described in connection with the specirlc details of -a preferred embodiment thereof, it must be under-'I stood that these, details are not intended rto be limitative of the invention except in so far as set forth in the accompanying claims. l

Having thus described my invention I declare that what I claim is:

l. In combination, an R. C. timing circuit, thermionic means arranged to receive va control voltage from said circuit and having 'a plate circuit including a responsive load device actuable when the plate current of said means is built up toa definite actuating degree after a desired period of time, a second thermionic means having a loaded plate circuit, control means in iirst said thermionic means connected to the plate of the second thermionic means a variable source of voltage arranged to energize said thermionic means, actuable means `for 'applying said source voltage across said R. C'. circuit, a control means -for the second thermionic means including a bias battery and a voltage divider connected across said source, in series with said battery, whereby the plate current of lthe second thermionic ymeans is low and the control voltage it applies to the rst thermionic means is high when the voltage of the source is low, said elements being correlated whereby the period is substantially uniform over a wide range of variation of the voltage ofthe source.

2. In combination, an R. C. timing circuit,

thermionic means having a control grid con-` put circuit whereby the voltage of 'the secondV control grid is diminished with increase of the-` second output circuit, a source yof direct current of variable voltage energizing said thermionic means, actuable means for connecting said source across said R. C. circuit, a negative bias battery connected to the third control grid and a voltage divider connected across said source applying a predetermined fraction of the source voltage to the positive terminal 'olf said battery, whereby the voltage applied to the second Vcontrol grid is greater when the supply voltage is low, thus compensating for the build-up or" the output voltage of the first thermionic means due to low voltage supply, whereby said period is made substantially uniform yfor wide variation of the supply voltage.

3. In combination, an R. C. timing circuit comprising a capacitor and a high resistor in series, means normally connecting the capacitor across a source of direct current of variable voltage, actuable means for connecting the condenser and resistor to said source with opposite polarity whereby the condenser discharges and recharges in the opposite direction, a thermionic means energized by said source and having a control grid connected to 'an intermediate point 'of said circuit and a plate circuit including a responsive load device actuable when theplate current builds up to an actuating degree after a period of time, which period of time tends to increase with lowering of the voltage of the source, fa second thermionic means energized by said source and including a control grid and a loaded plate circuit whereby its plate lvoltage drops With increase of plate current, an additional control grid in the r'st thermionic means connected to the plate of the second thermionic means, 'adjustable means associated with said source for increasing the voltage of the control grid of the second thermionic means when the source voltage is high, increasing the plate current of the second thermionic means and lowering the voltage of the additional control grid of the -rst thermionic means, whereby the voltage of the additional control grid of the first thermionic means compensates for the tendency of the period to vary with variation of the Voltage of the source.

4. In combination, an R. C. timing circuit comprising a capacitor and a high resistor in series, means normally connecting the capacitor across Va source of direct current of 'variable Voltage, actuable means for `connecting the condenser and resistor vto said source with opposite polarity whereby the condenser discharges and recharges in the opposite direction, a thermionic means energized by said source and having a control grid connected to an intermediate point of said ycircuit and a :plate circuit including a responsive load device actuable when the plate current builds up to an actuating degree after a period of time, which period of time tends to increase with lowering of the voltage of the source, a second thermionic means lenergized by said source and including a `control grid and a loaded plate vcircuit whereby its plate voltage drops with increase lof plate current, an additional control grid in the iirst thermionic means connected t'o the plate of the second thermionic means, means for applying a bias 'to the control grid 'of the vsecond thermionic means including a negative bias battery and a Voltage divider connected across the source for supplying a positive voltage to said battery, whereby the voltage of the control grid of the second thermionic means is increased by increase of voltage of the source `and the voltage applied to the additional control grid of the rst thermionic means is lowered, whereby said period of time is rendered substan tially uniform with wide variation of the voltage of the source.

5. In combination, an R. C. timing circuit, thermionic means having a control grid connected to an intermediate point of said circuit, a second control grid, and a, plate circuit including a responsive load device, a second thermionic means having a plate and a loaded plate circuit, a conductive connection between the plate of the second thermionic means and the second grid, a source of direct current of variable voltage energizing both thermionic means, and actuable means for connecting said R. C. circuit across said source.

6. In combination, an R. C'. timing circuit, thermionic means having a control grid connected to an intermediate point of said circuit, a second control grid, and a plate circuit including a responsive load device, a second thermionic means having a plate and a loaded plate circuit, and a third control grid, a conductive connection between the plate of the second thermionic means and the second grid, a source of direct current of variable voltage energizing both thermionic means, actuable means for connecting said R. C. circuit across said source, and means for supplying bias to the third control grid including a battery and means in series with the battery for supplying an opposed voltage from said source of magnitude depending upon the voltage of said source.

'7. In combination, an R. C. timing circuit, thermionic means having a control grid connected to an intermediate point of said circuit, a second control grid, and a plate circuit including a responsive load device, a second thermionic means having a plate and e, loaded plate circuit, and a third control grid, a conductive connection between the plate of the second thermionic means and the second grid, a source of direct current of variable voltage energizing both thermionic means, actuable means for connecting said R. C. circuit across said source, and .mea-ns for supplying bias to the third control grid including a battery having its negative terminal connected to the third grid and a voltage divider across said source having an intermediate point connected to the positive terminal of said battery.

HENRY M. MERRILL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,203,468 Martin June 4, 1940 2,227,490 Draper J an. '7, 1941 2,327,791 Hopper Aug. 24, 1943 

